The principals and effects of "generic leakage" have been demonstrated throughout the integrated circuit industry. A primary instigator of generic leakage has been found to be due to residual charges which become trapped along the uppermost surface of an integrated circuit or the passivation layer. Integrated circuit passivants, such as plasma nitride, are generally non-conductive materials which are highly resistant to penetration from external contaminants. The passivation layer protects the circuits of the integrated circuit from chemical effects of these external materials which can dramatically reduce the operating life and performance of the integrated circuit devices. The passivation is generally non-conductive to prevent unintentional leakage paths from forming between circuit elements on the integrated circuit, in particular, between metal pads and interconnecting runners.
On the surface of a typical integrated circuit, P-type resistors and devices may be formed within lightly N-doped epitaxial pockets which are electrically separated from surrounding using standard reverse biased diode techniques as illustrated in FIGS. 1, 2 and 3. However, the potential created by either accumulated charges in the molding compound (used in packaging the integrated circuit) or on top of an integrated circuit passivation layer can be enough to invert the surface of the epitaxy thereby creating a leakage path across the pocket to whatever the insulation well is tied to. This classic example of generic leakage phenomenon is illustrated in FIGS. 4-6. The generic leakage phenomenon can be present in hybrid/cavity (vacuum) encapsulated integrated circuits and applications.
Once an integrated circuit has been encapsulated within a package, usually in the form of an epoxy-based compound, it is protected from the environmental elements outside the boundaries of the package. Although epoxy molding compounds are somewhat penetrable by humidity and temperature over time, the plasma nitride passivation protects integrated circuit elements from any material defused with the moisture. These contaminants are generally ionic in nature, such as chlorine. In addition, the molding compound itself generally contains ionic materials such as bromine, etc., which are used as a package fire retardant.
At room temperature or under normal electrical conditions, these ionic contaminants remain stable and evenly dispersed within the epoxy or packaging material. Net charge distribution can be initially considered relatively neutral. However, when the molding compound is at elevated temperatures, it is very prone to external charge transfer and internal polarization, due to surface triboelectric charging (i.e., hot air movement from a handler) or nearby electric fields from charged surfaces or by the molding process. Often the device experiences increased temperatures above 150.degree. C. for example; due to high temperature ambient operations, or heat generated when operating the device, or effects of infrared (IR) solder reflow during system assembly, or by other methods of heating the device. As the device experiences elevated temperatures, the ions contained within the package tend to become freely mobile. These ions can be attracted to the surface of the integrated circuit by fields generated from electric current passing through the integrated circuit or by external fields, such as those generated in burn-in chambers or test handlers. As mentioned before, such a source of external field may be produced by circulating very hot, dry air which promotes significant levels of charge transfer and creates unwanted E-fields in the ambient environment. Eventually, ionic charges accumulate on the interface of the molding compound and the integrated circuit passivation. When like charges combine with increasing overall magnitude, a potential is distributed across the surface of the integrated circuit. As the package cools down, either from removal of power and/or reduction in temperature, the charges tend to "lock" into place. Both the molding compound and the passivation surface are non-conductive, and therefore the trapped charges have no place to leak off. FIG. 7 shows an integrated circuit with a package which has sustained a cool-down/lock-in phenomenon and has accumulated a negative charge on the top of its passivating layer.
Another situation which can produce unwanted charge on a passivation surface is "delamination." This can occur when the package material separates itself from the surface of the integrated circuit. This separation usually occurs through cycles of heating and cooling and leaves residual charges due to triboelectric differences in the materials. It has been discovered that a negative charge is left on the passivation layer such as plasma nitride, while positive charge resides on the packaging materials such as epoxy. This phenomenon is more prevalent with large integrated circuits, which inherently have higher amounts of encapsulated related stress due to longer body diagonal dimensions causing delamination at the corners of the die body that is encapsulated.
Thus, a means is needed to protect an underlying lightly doped epitaxial region of an integrated circuit chip from inverting due to charges either trapped in the packaging material or on the passivation layer. Further, there is a need for a means for "curing" a defective device which has been discovered to have charges trapped either in the molding compound or on the passivation layer.
Adequate field-plating of every inversion or generic leakage susceptible circuit element is not usually practical in most integrated circuit designs. Dual-level metal helps in this situation, by allowing another metallic layer with which to field plate while circuit signals are carried around a sensitive device. However, many designs simply cannot tolerate the area increase needed to manage adequate protection to all sensitive elements of the integrated circuit. In BiCMOS applications, polysilicon might be used to provide field-plate protection. However, none of these approaches attack the root cause of the problem, which is accumulated charge and increasing static potential on the surfaces of the molding compound and on the top surface of the integrated circuit passivation. Indeed, this affects not only generic leakage sensitive circuits, but creates variations in MOS device parameters, such as thresholds, the sheet resistance of lightly-doped poly or implanted resistors, and many other device parameters which influence an integrated circuit's performance criteria.